1. Field of the Invention
The invention relates to fabricating an article comprising a refractory dielectric body, more particularly to fabricating glass optical fiber preforms.
2. Discussion of the Related Art
As the use of optical fiber has increased, the demand for stronger, more durable fibers with improved optical properties has similarly grown. Loss mechanisms and structural faults in optical fiber typically result from imperfections and impurities existing in the glass preform from which the fiber is drawn, and much effort has gone into finding ways to reduce, remove, or eliminate these imperfections and impurities. Techniques for removing such imperfections and impurities include mechanical milling and chemical etching, neither of which is particularly desirable. Mechanical milling often introduces mechanical stresses into a preform that lead to crack formation, and chemical etching, while typically removing the intended imperfections and impurities, often introduces chemical by-products as new impurities. It is also known to use a plasma torch to remove surface impurities and imperfections, as discussed in U.S. Pat. No. 5,000,771, the disclosure of which is herein incorporated by reference. Each of these methods, however, essentially relies on removing a surface portion of the glass preform, at which the highest concentration of imperfections and impurities are normally found.
Yet, bulk impurities are also found in glass bodies. (Bulk indicates that the impurities are found throughout the glass material, e.g., both at the surface and into the interior of a body, as opposed to impurities that primarily lie within a few microns of the surface.) Typical bulk impurities in silica glass include chlorine and hydroxyl. Both types of impurities are known to induce detrimental bubbling during the manufacture of and drawing of fiber from silica glass preforms. Chlorine is typically introduced during fabrication of synthetic glasses from chlorine-containing compounds, e.g., from SiCl.sub.4, and during a subsequent purification step of such glasses in chlorine gas. It is possible for substantial amounts of chlorine to be introduced. For example, 1000 ppm of chlorine atoms in silica glass is not uncommon. Hydroxyl groups are introduced into silica glass bodies due to the presence of water in sol-gel and other fabrication processes, and due to the common use of oxy-hydrogen torches. Even natural fused quartz glass will contain hydroxyl impurities, although such glass typically does not contain high levels of chlorine.
In some fabrication processes, it would be desirable to leave the surface of a glass body substantially intact, yet still reduce surface impurities, and advantageously bulk impurities as well. Thus, a method for fabricating an article comprising a refractory dielectric body, e.g., an optical fiber preform, is desired in which both surface and bulk impurities are reduced, with substantially no removal of the surface of the body.